The present invention relates to waste energy or heat recovery system for use with controlled atmosphere environments, such as a controlled atmosphere brazing system. The invention particularly pertains to a system for recovering heat from the heated sections of the brazing system for use in a dry-off oven.
Brazing is a commonly used technique for joining metal parts with close fitting joints. Typically, a flux material is disposed at the location of the joint and then melted within a furnace or oven to flow into the gap between adjacent parts. Most commercial brazing operations are carried out on a continuous conveyor belt that passes through heated sections, or furnaces, of the brazing system. The furnaces are usually fitted with a muffle disposed within a refractory structure. The muffle is heated by the use of natural gas and/or electric heating elements outside the muffle. The brazing environment within the muffle is maintained as a controlled atmosphere, meaning that the atmosphere is maintained to facilitate the brazing process and to prevent oxidation or coloration. Typically, the controlled atmosphere is maintained by continuously pumping nitrogen into the muffle.
In order to ensure an optimum braze, it is first necessary to eliminate any moisture from the metal parts of the flux. Thus, most brazing systems include a dry-off or dehydration oven between the fluxer and the controlled atmosphere furnaces. The purpose of the dry-off oven is to raise the core part temperature sufficiently high to evaporation off all moisture from the part. Typically, a dry-off oven will raise the part temperature to 150xc2x0 C. (350xc2x0 F.) in an air atmosphere.
Thus, it can be appreciated that the standard continuous belt brazing system will include a number of ovens or furnaces that include their own heating element(s). The braze furnace will often be heated by a combination of gas fired zones and electric zones. Where a pre-heater is employed to increase the flux and core temperature, the pre-heater furnace will usually be gas fired, but may also include additional electrically heated zones. Finally, the conventional dry-off oven can be gas fired and/or electrically heated. Each one of these units produces hot gas and products of combustion that must be exhausted to the outside of the building housing the brazing system. These hot gases require special handling, such as alloy ducts, insulated ducts and high temperature exhaust fans. Moreover, energy contained within these hot gases is lost to the atmosphere.
Much effort has been expended to make the brazing process more efficient and to reduce the overall energy requirements for the brazing system. More efficient gas-fired or electrical heating elements have reduced the fuel requirements and provided more efficient heating of the various sections of the brazing system. Improved venting systems are better able to discharge the waste gases from the various brazing system sections. However, there remains a need for even greater improvements to the heating of the brazing system and to the handling of the waste gases associated with the system.
In order to address these needs, the present invention contemplates a waste energy, or heat, recovery system that is integrated into a serial closed-atmosphere process. A system of pipes extract hot gas from the heated downstream components of the serial process and feed them to heated components at the upstream end of the process. This recycled hot gas provides a portion or even all of the heating requirements for the upstream heated component. In one embodiment, the recovery system extracts hot gas from each downstream heated component, and returns that hot gas to the upstream component. It should be understood that the recovery system can redirect hot gas from any component of the system to any other component, not just simply from downstream component to upstream component.
In another aspect, the recovery system extracts all of the hot gas from the downstream components and circulates all of the hot gas through the upstream component. In order to modulate or control the temperature of this upstream component, ambient air can be fed into the upstream component to mix with the recycled hot gas. A temperature sensor can be provided at the upstream component to monitor the temperature and regulate the inflow of ambient air to mix with the recycled hot gas.
In one feature of the invention, the waste energy recovery system of the present invention is integrated into a continuous controlled atmosphere brazing system. The brazing system can include one or more downstream heated components, such as a brazing furnace or a pre-heat furnace. The brazing system also includes an upstream dry-off oven. Rather than provide the dry-off oven with its own heat source, the recovery system of the present invention recirculates the hot gas from each of the downstream components back to the interior of the dry-off oven. In one aspect, this recirculation can be accomplished by a series of tube extending along the process path and projecting into the heated portion of each downstream component. An insulated manifold can be disposed between adjacent heated components with the recirculation tubes from each component opening into the interior of the manifold.
The waste energy recovery system includes a flow device that draws the hot gas from the heated portion of each heated component of the brazing system and directs that hot gas through the upstream component. In certain embodiments, the flow device can include fans disposed within the upstream dry-off oven with the end of the recirculating tubes at the suction side of the fans. Thus, the fans continuously draw the hot gas from the downstream components. In addition, the fans can be sized and positioned to draw ambient air into the dry-off oven. In some embodiments, the ambient air is provided through the inlet to the oven and/or through an additional air inlet.
The air inlet into the upstream dry-off oven can be modulated by a control valve. The control valve can control entry of air into the oven at the suction side of the fans. In certain aspects, the control valve can be regulated by a temperature sensor disposed within the dry-off oven. The temperature signal from the sensor can be used by the control valve to close or open a valve within the airflow path into the oven, or to modulate the position of the valve and therefore the flow rate of ambient air. The valve can be a variable position butterfly valve within an air intake plenum.
In certain embodiments, the recirculation tubes can extend into the heated portions of the downstream heated components. Multiple tubes can be provided, with each tube extending into a different heating zone within the component. Similarly, at the upstream end, multiple tubes can extend from an upstream manifold into the dry-off oven. A greater number of tubes can exhaust at the intake end of the oven to more quickly raise the part and flux temperature.
It is one object of the present invention to provide a system for recovery and using waste energy or heat energy from a process that would otherwise be exhausted form the process. In the context of a continuous brazing system, it is an object of the invention to utilize the hot gases generated in heating a brazing furnace and/or a pre-heater.
One benefit of the present invention is that is can significantly reduce the energy requirements for a continuous heated process. More specific to a continuous brazing system, the present invention beneficially allows the use of a xe2x80x9cburnerlessxe2x80x9d dry-off oven.
Another benefit achieved by the recovery system of the present invention is that it reduces the requirements for exhausting hot gas outside the process facility. These reduced requirements can translate into lower cost for building the process facility, as well as reduced environmental effects.
Other objects and benefits of the present invention will become apparent upon consideration of the following written description, taken together with the accompanying figures.